In addition to a compression ring for keeping combustion gas out, an oil ring for controlling oil on a cylinder inner surface is mounted on a piston of a reciprocating engine (an internal combustion engine). As such an oil ring, a multi-piece oil ring in which one or a pair of side rails are attached to a space expander in an annular shape has been popularly used.
The side rail used in the multi-piece oil ring is formed in a split ring shape with an opening such that, as being pushed by the space expander, the side rail extends its diameter and its outer peripheral surface contacts with the cylinder inner surface applying a predetermined contact pressure (a predetermined surface pressure). When the piston reciprocates during running of the engine, the outer peripheral surface of the side rail slides on the cylinder inner surface, forming an oil film with an appropriate thickness on the cylinder inner surface and scraping off excess oil adhered to the cylinder inner surface toward a crankcase to prevent the oil from climbing up to a combustion chamber.
In recent years, following an improvement in functionality of the internal combustion engine to meet a market demand for low fuel consumption and low oil consumption, there has been a demand for a multi-piece oil ring capable of reducing friction against the cylinder inner surface as well as the oil consumption by controlling an oil scraping-up action during piston upstroke (compression stroke and exhaust stroke) and amplifying an oil scraping-off action during piston downstroke (intake stroke and combustion stroke). In an effort to meet such a demand, there are proposed side rails having outer peripheral surfaces in various shapes facing radially outward.
For example, PLT 1 set forth below describes a side rail having an outer peripheral surface facing radially outward formed in a curved surface having a vertex at an axial center of the outer peripheral surface and protruding radially outward.